Why does sculpting with dyntopo produces small cracks on the mesh surface?
The first screenshot shows big amount of faces stretched horizontally really much, probably as a result of dyntopo atop of already subdivided mesh with maybe not proper base topology. They are bulging out of the model, most likely as a result of the brush drawing.

In short, don't use dyntopo in this case. It is useful for rough and quick adding details when e.g. base and draft sculpting is done; it's possible to do the final mesh with it only as well, but not mixing it with Subsurf or any other type of subdivision. The reason to use dyntopo is to selectively subdivide mesh where needed and sculpt it at the same time. On the other hand in the case shown you have big amount of geometry which as you see will prevent you from easy editing. Further, it's mostly useless as much less geometry is pretty much enough for showing details. 

The thing also to keep in mind is that if you need more details continue subdividing the mesh relatively equal for all its surface. There are plenty of tutorials about subdivision while sculpting which will show you that in more details.

The easiest solution is to undo dyntopo (using backup copy of the mesh is assumed) and go with Multires only (instead of Subsurf). Then it will be possible to go with clean topology as much as possible while leaving base mesh relatively simple (Multires will do that for you).

If dyntopo is undoable then it's possible to use it for unsubdividing as well, just enable Collapse in the Dyntopo rollout and continue scultping on the mesh while checking what geometry it generates.